The use of electrochemical (EC) detectors for liquid chromatography (LC) is a growing field. The primary use of such detectors is for trace analysis offering up to four orders of magnitude increased sensitivity over the commonly used ultraviolet-visible (UV-VIS) photometer. Other possible advantages of EC detectors include increased selectivity, similar response to equal concentrations of electroactive species and relatively inexpensive hardware.
Despite these apparent advantages, EC detectors have not achieved widespread routine use for a number of reasons. Response is achieved by electrochemically oxidizing or reducing electroactive species in the chromatographic eluent on the cell working electrode (WE) at potentials characteristic of the species. The current that flows at a given potential is proportional to the sample concentration and the hydrodynamic pattern of the cell. Many compounds yield reaction products which form passivating films on the WE surface thereby changing detector response with time. Such fouling can be quite rapid, allowing as little as four hours of operation with continuously decreasing response, after which the cell must be disassembled and the WE surface renewed. Reassembly and equilibration back to near steady state background current may then require as much as 24 hours before trace analysis can resume. Continuously changing response and extensive skilled maintenance are very unattractive and expensive to the routine user. A number of schemes have been attempted to prevent or minimize fouling. Mechanical scrapers produce noisy signals due to variations in the exposed surface area of the electrode and compromise the response volume requirements of the cell. A dropping mercury electrode is difficult to miniaturize and due to the ease of oxidation of mercury is really only useful for electrochemical reductions. Both techniques attempt to mechanically renew the electrode surface on a short time scale compared to the analysis time.
Continuous calibration with internal standards does not solve the problem but does allow reasonably accurate analysis during the relatively short lifetime of the electrode surface. As the electrode fouls, the signal to noise ratio falls and there is an increased chance of sample contamination with addition of the standard.
Very low concentrations of normally fouling species tend not to foul due to the low surface coverage and lack of interaction on the electrode surface. Fouling may then be prevented if extensive sample pretreatment removes or greatly dilutes offending species without substantially changing the species of interest. This may require substantial method development, create sample recovery or contamination problems, and increase analysis time and cost.
As demonstrated by Fleet in U.S. Pat. No. 4,059,406 issued Nov. 22, 1977, other workers have attempted to desorb offending species before the surface coverage built up to critical levels by changing the potential of the working electrode periodically to one where the specie desorbs. Unfortunately, many troublesome chemical species are very strongly adsorbed and do not significantly desorb at potentials where the electrode and/or the solution is not strongly oxidized or reduced. In addition the kinetics of such desorption may be too slow to be of practical use in cleaning and such a desorption method is very specific so a new method must be developed for each set of analytical conditions. The use of such a method with large electrodes requires relatively high power electronics to charge the WE capacitance when large potential steps are made.
Any EC detector should be optimized in three ways. The chromatographic peaks should not be substantially diluted or distorted by the cell, the electrode configuration should yield undistorted electrochemical performance and the cell internal geometry should produce an optimum undistorted hydrodynamic pattern maximizing response at LC flow rates. Prior EC LC detectors have not satisfied all of these requirements.
Optimum LC conditions for trace analysis are somewhat different than those used for more routine higher concentration analysis. Separations should be optimized to achieve very small values of the column solute capacity factor (i.e., k' should be approximately 0.5-1.5) with isocratic conditions to minimize sample dilution and background slope. The column should use the smallest diameter packing material available, the largest internal diameter (ID), and the shortest length consistent with the resolution required and sample size available. The volumetric flow rate through the column should provide the highest linear velocity consistent with the resolution required and the pumping system pressure capability. The injection volume should be the maximum consistent with the column capacity.
Under these conditions conventional 4.6 millimeters ID columns with 3 micrometer diameter column packing operated at optimum linear velocity should allow complete analysis of up to 24 components in less than 4.5 minutes with the widest peak 6.25 seconds across the base. By increasing linear velocity to 5 times optimum, the widest peak drops to 1.6 seconds across the base with a peak volume of 0.12 milliliters. Analysis time also drops by a factor of 5 to less than one minute. The fastest peak will then be less than 1 second across the base with a peak volume less than 80 microliters.
Smaller diameter columns are an important new area of LC technology. The primary advantages of such columns are much lower solvent consumption and much smaller sample size requirements. Such columns perform the same as conventional columns except peak volumes of even the longest retained peak can be as small as 4 microliters.
In order for an electrochemical detector to follow such an analysis without severe attenuation or peak distortion, the cell response volume should be in the range 0.1 to 8 microliters depending on the column used, and the detector time constant should be less than 200 milliseconds. Further, detector sensitivity should not be compromised to achieve such a fast response time.
The greatest effort in LC detector cell design has been placed in minimizing cell response volume and cells have been built with response volumes as low as 1 nanoliter. Achieving low dead volume is made difficult by the fact that it is often impossible to machine components of the sizes required since they are below the tolerances of machine tools. Fabrication techniques and cell geometries have evolved which achieve the desired response volume but at the expense of electrochemical and/or hydrodynamic performance.